First, you do not need a child theme to implement Custom CSS.
Custom CSS can be placed in two different location.
For global rules that you create with a full selector, place the rule in Main options > Fonts & Custom > Custom CSS Rule box.
For additional styling of an element that has and existing options box, place the CSS property between curly brackets in the object attribute CSS+ box
All these are saved with all other theme options and survive any update.
You only need a child theme if you have advanced PHP modifications to make to the theme.
So Place your Custom CSS there and if it does not work, tell us what is the Custom CSS and describe precisely what it is for so we can check the page and see what is going on.